.NET Assembly Decompilation with ILSpy

Use this skill to understand how .NET code works internally by decompiling compiled assemblies.

Prerequisites

• .NET SDK installed
• ILSpy command-line tool available via one of the following:
  • dotnet dnx ilspycmd (if available in your SDK or runtime)
  • dotnet tool install --global ilspycmd

Both forms are shown below. Use the one that works in your environment.

Note: ILSpyCmd options may vary slightly by version. Always verify supported flags with ilspycmd -h.

Quick start

# Decompile an assembly to stdout
ilspycmd MyLibrary.dll
# or
dotnet dnx ilspycmd MyLibrary.dll

# Decompile to an output folder
ilspycmd -o output-folder MyLibrary.dll

Common .NET Assembly Locations

NuGet packages

~/.nuget/packages/<package-name>/<version>/lib/<tfm>/

.NET runtime libraries

dotnet --list-runtimes

.NET SDK reference assemblies

dotnet --list-sdks

Reference assemblies do not contain implementations.

Project build output

./bin/Debug/net8.0/<AssemblyName>.dll
./bin/Release/net8.0/publish/<AssemblyName>.dll

Core workflow

1. Identify what you want to understand
2. Locate the assembly
3. List types to find the target
4. Decompile the target

Commands

List types in an assembly

Use -l with an entity type to discover what's inside:

# List classes
ilspycmd -l c MyLibrary.dll

# List interfaces
ilspycmd -l i MyLibrary.dll

# List structs, delegates, enums
ilspycmd -l s MyLibrary.dll
ilspycmd -l d MyLibrary.dll
ilspycmd -l e MyLibrary.dll

Basic decompilation

ilspycmd MyLibrary.dll
ilspycmd -o ./decompiled MyLibrary.dll
ilspycmd -p -o ./project MyLibrary.dll

Targeted decompilation

# Decompile specific type
ilspycmd -t Namespace.ClassName MyLibrary.dll

# Specify C# language version (default: Latest)
ilspycmd -lv CSharp12_0 MyLibrary.dll

View IL code

ilspycmd -il MyLibrary.dll

Clean output

# Remove dead code and dead stores for cleaner results
ilspycmd --no-dead-code --no-dead-stores MyLibrary.dll

Organized project output

# Decompile with nested directories matching namespaces
ilspycmd --nested-directories -p -o ./decompiled MyLibrary.dll

References

• references/diagrammer.md — Generate interactive HTML diagrams from assemblies. Load when the user asks to visualize or explore type relationships.

Notes on modern .NET builds

• ReadyToRun images may reduce readability
• Trimmed or AOT builds may omit code
• Prefer non-trimmed builds

Legal note

Decompiling assemblies may be subject to license restrictions.